This invention relates generally to a method of coating optical glass fibers and in particular to such a method that is carried out immediately after the fiber is formed which results in reducing abrasion to the fiber and interaction of the environment with the fiber, and increasing the life time of the fiber during both storage and use.
Glass optical fibers when initially formed have high tensile strengths. After extended periods of use or storage the optical fibers may break when subjected to tensile stresses substantially lower than the original tensile strength rating of the fiber. One reason for this breakage, known as static fatigue, is the development of surface imperfections along the glass outer perimeter which form microcracks. This fatigue is at least in part attributable to the presence of water molecules and hydroxyl groups on the glass surface of the fibers. The water attacks the surface and causes formation of weak bonds which are broken by applied stress.
With glass-on-glass optical fibers where the core comprises a glass material and the cladding comprises a glass material, the presence of water molecules on the outer glass surface tends to cause the glass structure on the outer surface to become substantially weakened over a period of time so that the fiber ruptures under stress forces that would be incapable of causing the fibers to fracture in the absence of water or water vapor.
In the fiber forming operation, for example, fiber drawing, the glass-on-glass optical fiber is frequently coated with a polymer, such as a silicone resin immediately after formation in order to preserve its pristine strength and for handling ease. Although the silicone material is effective to prevent dust particles from contacting the outer glass surface, the silicone material is relatively pervious to water. Over a period of exposure in air, at ordinary concentrations of water vapor, water is able to permeate through the silicone layer and to interact with the outer glass surface, with the above-mentioned deleterious consequences.
The same mechanism of static fatigue occurs with plastic clad optical fibers where the core material comprises silica or other glass and the cladding comprises a silicone material or other polymer.
One method that has been employed in each of these fibers to overcome the problem of water penetration is the application of a thermoplastic resin over the silicone material. Although the extruded thermoplastic jacket covering the silicone material reduces the penetration of water through the silicone layer to some degree, water still can permeate through the jacket to the silicone material and from there to the glass surface of the core, again resulting in degradation of the core glass strength.
An alternative to using the silicone process is sealing the drawn fiber surface from the surrounding environment by coating the initially formed fiber with a metallic material, such as aluminum or nickel, or with a dielectric material, such as silicon nitride or tin oxide. Such sealing can be performed by using chemical vapor deposition (CVD). However, the CVD process has several disadvantages such as impingement of coating particles that are formed in a gaseous medium against the pristine glass fiber to produce surface damage, non-uniform coating thickness and large grains of coating material.
The interaction of coating particles with the fiber surface can be eliminated and an initially hermetic coating on the fiber may be obtained by means of a heterogeneous nucleation thermochemical deposition (HNTD) process thus preventing the degradation of fiber strength over the lifetime of the fiber. The HNTD process may be used to apply either a metallic or a dielectric coating to the fiber surface. Important considerations in using the HNTD process are the surface temperature of the fiber and the deposition time. While the surface temperature can be maintained adequately, the thickness of the primary coating is usually thin, that is, much less than one micron for a fiber draw speed of approximately 20-40 meters per minute. A thicker coating is preferable for maintaining hermeticity.